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@@ -130,10 +130,10 @@ Dedicated to PL group at Indiana University.
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The $S_0$ language includes integers, operations on integers,
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(arithmetic and input), and local variable definitions. This language
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is rich enough to exhibit several compilation techniques but simple
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-enough so that we can reasonably hope to implement the compilation to
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-x86-64 assembly in two weeks of hard work. The instructor solution
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+enough so that it is reasonable to implement the compilation to
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+x86-64 assembly in two weeks of part-time work. The instructor solution
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for the $S_0$ compiler consists of 6 recursive functions and a few
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-small helper functions that together span 256 lines of code.
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+helper functions that together span 256 lines of code.
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The syntax of the $S_0$ language is defined by the following grammar.
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\[
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@@ -159,13 +159,17 @@ Given user input of 2, the following expression evaluates to $5$.
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(+ \; (\key{read}) \; 3) \Longrightarrow 5
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\]
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Given user input of 2 then 3, the following expression may evaluate to
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-either $1$ or $-1$ depending on the whims of the Scheme
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-implementation.
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+either $1$ or $-1$ depending on whether the implementation (an interpreter
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+or compiler) decides to evaluate the \texttt{read} on the left before
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+the \texttt{read} on the right, or vice versa.
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\[
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(+ \; (\key{read}) \; (-\; (\key{read})))
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\Longrightarrow
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1 \text{ or } -1
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\]
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+We include the \texttt{read} operation in $S_0$ to demonstrate that
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+order of evaluation sometimes makes a difference and also to prevent
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+the use of an interpreter to trivially implement the compiler for $S_0$.
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\section{x86-64 Assembly}
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jsiek